A glass substrate that is used in a display device, a frame, a handicraft, a vessel and the like has various advantages such as small linear expansion coefficient, excellent gas barrier property, high optical transmissivity and surface flatness, excellent heat resistance and chemical resistance and the like, but has disadvantages because it is easily broken due to weakness to impact and heavy due to high density.
Currently, an interest for liquid crystals, organic light emitting display devices, and electronic paper is rapidly growing and many studies for replacing glass used as a substrate of the display device by plastic have been made. That is, if the glass substrate is replaced by the plastic substrate, the total weight of the display device becomes light, flexibility of design may be provided, and it is strong in respects to an impact, and in the case of when it is manufactured by using a continuous process, it may have economic efficiency as compared to the glass substrate.
Meanwhile, in order to use it as the plastic substrate of the display device, properties such as a high glass transition temperature that is capable of enduring a process temperature and a deposition temperature of a transparent electrode, oxygen and steam blocking property for preventing aging of liquid crystal and an organic light emitting material, a small linear expansion coefficient and dimensional stability for preventing distortion of a substrate according to a change in the process temperature, high mechanical strength that has high compatibility in respects to a process equipment used in a known glass substrate, chemical resistance that is capable of enduring an etching process, high optical transmissivity, small birefringence, and scratch resistance of a surface are required.
However, since there is no high performance polymer base film (including a polymer film and a polymer-inorganic material complex film) that satisfies all of these conditions, an effort for satisfying the above physical properties by performing functional coating several layers on the polymer base film has been made.
As an example of the representative coating layer, there are an organic flattening layer that provides flatness by lowering defects of the polymer surface, a gas barrier layer that includes an inorganic material for blocking gas such as oxygen and steam, and an organic or organic-inorganic hard coating layer for providing scratch resistance of the surface.
In the case of known many multilayered plastic substrates, they are subjected to a process for coating an inorganic material gas barrier layer on a polymer base and forming a hard coating layer on the gas barrier layer. When they are manufactured as the multilayered structure, there are problems in that the polymer base may be deformed, cracks may be generated on the inorganic thin film, and stripping may occur because there is a large difference in the linear expansion coefficients between the polymer base and the gas barrier layer. Accordingly, an attachment property between coating layers are very important.
Several organic-inorganic layers are manufactured by repeating a process for forming a monomer thin film on a polymer base film manufactured by Vitex Systems, Co., Ltd. in the USA, irradiating UV thereonto to perform the polymerization reaction such that polymerization is carried out (solidified organic layer), and shaping an inorganic thin film by using a sputtering method, and a flexible substrate that has excellent gas barrier property is manufactured.
However, even though a product that has excellent gas barrier property may be obtained by the above method, it is not useful to displays requiring low linear expansion coefficient, and they cannot suggest any solution in respects to this.
U.S. Pat. No. 6,465,953 discloses a method for dispersing getter particles that are capable of being reacted with inflow oxygen and steam on a plastic substrate in order to use the plastic substrate in an organic light emitting device that is sensitive to oxygen and steam. The size of the getter particle should be sufficiently smaller than the size of a specific wavelength of the emitted light and the particles should be uniformly dispersed so that emitted light is not scattered but penetrate the substrate. In addition, this method minimizes the amount of oxygen and steam that flow thereinto by coating a gas barrier film made of the inorganic material on the plastic substrate. However, in the above method, it is difficult to manufacture the substrate while nanoparticles having the size of 100 to 200 nm are uniformly dispersed, the thickness of the plastic substrate should be large in order to include a great amount of getter particles that are capable of being reacted with oxygen and steam, and the inorganic material gas barrier film is directly coated on the plastic substrate, such that cracks or stripping may occur on the gas barrier film because of a temperature change.
U.S. Pat. No. 6,322,860 discloses that a plastic substrate for displays is manufactured by coating a composition (a multifunctional acrylate-based monomer or oligomer, alkoxysilane, and a mixture thereof) that includes silica particles and the like and is capable of being crosslinked on one side or both sides of a polyglutimide sheet that has a thickness within 1 mm in some cases, photocuring or thermal curing it to manufacture a cross-linked coating film, coating a gas barrier film thereon, and in some cases, coating the cross-linked coating film on the barrier film. However, in the above method, only in the case of special cases, the oxygen transmissivity and the steam transmissivity are small enough to be used for liquid crystal display devices, but the low linear expansion coefficient and excellent dimensional stability that are necessarily required in order to use it instead of the glass substrate are not improved.
U.S. Pat. No. 6,503,634 discloses a multilayered film in which the oxygen transmissivity is 1/30or less of the polymer base before the coating is carried out and the steam transmissivity is 1/40or less of the polymer base before the coating is carried out by coating ORMOCER that is an organic-inorganic hybrid and a silicon oxide layer on one polymer base or an intermediate layer of two polymer bases. However, the above method suggests that it can be used as a material for packing material because the oxygen and steam transmissivities are largely lowered as compared to the polymer base before the coating is carried out, but does not mention improvement in linear expansion coefficient and dimensional stability.
Japanese Patent Publication No. 1998-016142 discloses a gas barrier layering film in which a metal oxide layer, a ceramic layer that is formed from polysilazane, and a cured layer that is formed by using partial hydrolysis materials of alkoxysilane are sequentially layered. However, in this case, since the metal oxide is directly layered on the polymer film, there is a problem that is caused by a difference in linear expansion coefficient.